1. Field of the Invention
The present invention relates to a process for forming metal patterns on a substrate where a thermally depolymerizable polymer is used as a lift-off or release layer for a blanket metallization layer.
2. Description of the Prior Art
IBM Technical Disclosure Bulletin Vol. 22, No. 2, July 1979, pages 541 and 542 disclose, respectively, the use of gamma radiation treatment to enhance unzipping and the use of poly-.alpha.-methylstyrene and polymethylmethacrylate as unzippable polymers for green sheets.
IBM Technical Disclosure Vol. 15, No. 1, June 1972, page 174, discloses inter alia, polymethylmethacrylate electron beam resists where heating is used to bring out the image rather than developing in a solvent.
U.S. Pat. No. 3,535,137, Haller et al, discloses a method for manufacturing electron beam degradable, etch-resistant, positive resist masks from vinyl-type polymers and copolymers in which one-half the carbon atoms of the main chain are quaternary such as polymethyl methacrylate or cellulose derivatives such as cellulose acetate. Areas of the mask exposed to electron beam radiation are reduced in molecular weight and can be developed by an in situ fractionation step. Prior to irradiation the polymer film is baked to improve adhesion and handling characteristics.
U.S. Pat. No. 3,985,597 Zielinski discloses a wet lift-off system based upon the use of a solvent where polysulfone is dissolved out in N-methyl pyrrolidone.
U.S. Pat. No. 3,934,057, Moreau et al, relates to a high sensitivity positive resist and mask formation process. The resist comprises plural layers of resist coated on a substrate. Each resist layer includes a radiation degradable organic polymer and each successive resist layer has a lower dissolution rate in the resist developer than the resist layer which it overlies. Useful resists include vinyl type polymers such as lower alkyl esters of methacrylate, n-butylmethacrylate and t-butylmethacrylate, and diazo sensitized novolac resins.
U.S. Pat. No. 4,004,044 Franco et al discloses a wet method for forming patterned films utilizing a transparent lift-off mask where a photoresist layer is removed in a solvent such as N-methyl pyrrolidone.
U.S. Pat. No. 4,181,755 Liu et al discloses a wet method for generating thin film patterns by an inverse lift-off technique which involves soaking in an appropriate photoresist solvent.
U.S. Pat. No. 4,224,361 Romankiw discloses a high temperature lift-off technique.
U.S. Pat. No. 4,272,561 Rothman et al discloses a solvent-based wet lift-off technique used in the fabrication of integrated circuits.
U.S. Pat. No. 4,328,263 Kurahasi et al discloses a process for manufacturing semiconductor devices where a solvent-based wet lift-off technique is used.
The process of the present invention finds application in any of the above processes where a metallization layer is removed.
The present invention avoids the disadvantages of currently used wet procedures which involve exposure to strong solvents such as NMP for extended periods of time at elevated temperatures.
For example, per the prior art wet procedures, if a lift-off layer comprises a polysulfone, the same is typically removed by soaking in hot N-methyl pyrrolidone (NMP) for 8-18 hours at elevated temperatures, e.g., 85.degree.-130.degree. C. The solvent swells and dissolves the polymeric polysulfone layer and removes the overlying metal film, leaving behind the desired metallization pattern, e.g., metallized vias. Certain devices employ organic insulators in the structure such as a polyimide in place of the inorganic insulators. Problems encountered with this particular system using polysulfone as a lift-off layer include: (1) long soak times in hot NMP; (2) moderately polar, strong solvents such as NMP readily swell organic polymers such as polyimides, the resulting volume increase causes stressing and cracking of contiguous RIE barrier layers such as Si.sub.3 N.sub.4 ; (3) polymers such as polyimides readily absorb NMP even at room temperature, which is then extremely difficult to remove from the device and can lead to outgassing problems and result in ill-shaped metallurgy as well as corroded metallurgy; (4) polysulfone is sensitive to electron beam radiation which apparently leads to crosslinking, resulting in reduced solubility and requiring longer NMP soak times to effect lift-off, resulting in solvent-resistant residues; and (5) swelling can result in image distortion.
Polysulfones are also moisture sensitive, and their coating and registration characteristics change with humidity. This problem in addition to those noted above are not encountered with the depolymerizable polymers of the present invention.